Lithographic apparatus and device manufacturing method

ABSTRACT

A method for configuring an illumination source of a lithographic apparatus is presented. The method includes dividing the illumination source into pixel groups, each pixel group including one or more illumination source points; selecting an illumination shape to expose a pattern, the illumination shape formed with at least one pixel group; iteratively calculating a lithographic metric as a result of a change of state of a pixel group in the illumination source, the change of the state of the pixel group creating a modified illumination shape; and adjusting the illumination shape based on the iterative results of calculations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of U.S. patent applicationSer. No. 11/797,356 filed on May 2, 2007, which claims the benefit ofU.S. Provisional Patent Application No. 60/877,350, which was filed onDec. 28, 2006. The contents of these applications are herebyincorporated in their entirety by reference.

FIELD

The present invention relates to a lithographic apparatus and a devicemanufacturing method.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) of a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at once, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

Photolithography is widely recognized as one of the key steps in themanufacture of ICs and other devices and/or structures. At present, noalternative technology seems to provide the desired pattern architecturewith similar accuracy, speed, and economic productivity. However, as thedimensions of features made using photolithography become smaller,photolithography is becoming one of the most, if not the most, criticalgating factors for enabling miniature IC or other devices and/orstructures to be manufactured on a truly massive scale.

A theoretical estimate of the limits of pattern printing can be given bythe Rayleigh criterion for resolution as shown in equation (1):

$\begin{matrix}{{CD} = {k_{1}*\frac{\lambda}{{NA}_{PS}}}} & (1)\end{matrix}$where λ is the wavelength of the radiation used, NA_(PS) is thenumerical aperture of the projection system used to print the pattern,k₁ is a process dependent adjustment factor, also called the Rayleighconstant, and CD is the feature size of a feature arranged in an arraywith a 1:1 duty cycle (i.e., equal lines and spaces or holes with sizeequal to half the pitch). It follows from equation (1) that reduction ofthe minimum printable size of features can be obtained in three ways: byshortening the exposure wavelength λ, by increasing the numericalaperture NA_(PS) or by decreasing the value of k1.

In order to improve resolution performance of a lithographic system,various tools may be used. In one approach, an illumination system of alithographic apparatus is refined by considering alternatives to fullcircular illumination shapes. Such full circular illumination shapes arealso referred to as conventional illumination. A radiation system of alithographic apparatus generally includes an illumination system. Theillumination system receives radiation from a source, such as a laser,and produces an illumination beam for illuminating an object, such asthe patterning device (e.g. a mask on a mask table). Within a typicalillumination system, the beam is shaped and controlled such that at apupil plane of the illumination system the beam has a desired spatialintensity distribution. Such a spatial intensity distribution at thepupil plane effectively acts as a virtual radiation source for producingthe illumination beam. Various shapes of the intensity distribution,consisting of (substantially uniform) light areas on a dark background,can be used. Any such shape will be referred to, hereinafter, as anillumination shape, an illumination mode, an illumination configuration,an illumination setting or a shape of an illumination source. A maximumselectable extent of aforementioned virtual radiation source is definedby the design of the illumination system (e.g., the optical extent ofthe illumination pupil), and corresponds to a maximum clear aperturesize in the illumination pupil.

A system where illumination radiation is obliquely incident on thepatterning device at an angle so that the zero-th and first diffractionorders are distributed on alternative sides of the optical axis mayallow for improvements. Such an approach is generally referred to asoff-axis illumination. Off-axis illumination improves resolution byilluminating the patterning device with radiation that is at an angle tothe optical axis of the projection system. Examples of off-axisillumination include multipole illumination and annular illumination.The incidence of the radiation on the patterning device, which acts as adiffraction grating, improves the contrast of the image by transmittingmore of the diffracted orders through the projection system. Off-axisillumination techniques used with conventional masks produce resolutionenhancement effects similar to resolution enhancement effects obtainedwith phase shifting masks. Besides off-axis illumination, othercurrently available RET include optical proximity correction (OPC) ofoptical proximity errors (OPE), and sub-resolution assist features(SRAF). Each technique may be used alone, or in combination with othertechniques to enhance the resolution of the lithographic projectiontool.

As illumination systems have evolved from producing conventional toannular, and on to quadrupole and more complicated illuminationconfigurations, the control parameters have concurrently become morenumerous. In a conventional illumination mode, a circular area includingthe optical axis is illuminated in a pupil of the illumination system,the only adjustment to the illumination mode being to alter the outerradius (σ_(r)) of the circular illumination shape. Annular illuminationrequires the definition of an inner radius (σ_(c)) in order to definethe illuminated ring of the annular illumination mode. For multipolepatterns, the number of parameters which can be controlled continues toincrease. For example, in a quadrupole illumination configuration, inaddition to the two radii, a pole angle α defines the angle subtended byeach pole between the selected inner and outer radii.

Concurrently, patterning device (e.g., mask) technology has beenevolving as well. Binary intensity masks have given way to phase shiftmasks and other advanced designs. While a binary mask simply transmits,reflects or blocks imaging radiation at a given point, a phase shiftmask may attenuate some radiation or it may transmit or reflect thelight after imparting a phase shift, or both. Phase shift masks havebeen used in order to image features which are on the order of theimaging radiation's wavelength or smaller, since diffraction effects atthese resolutions can cause poor contrast and end-of-line errors, amongother problems.

Modern illumination systems have ever increasing numbers of variableswhich can be manipulated. In order to account for the variouspermutations of variable settings and to reduce the cost of trial anderror optimization of illumination configurations, photolithographicsimulations may be used to optimize the illumination conditions for agiven mask pattern.

One approach for determining an optimal combination of the illuminationshape and the patterning device pattern (e.g., the mask pattern) is tocalculate the normalized aerial image log slope (NILS) at a number ofpre-selected points, commonly referred to as fragmentation points, alongthe border of pattern features. Then, the intensity and shape of theillumination and the magnitude and phase of the diffraction orders fromthe patterning device pattern are simultaneously changed to form animage in the image plane that maximizes the minimum image log slope atthe fragmentation points while forcing the intensity at thefragmentation points to be within a predetermined intensity range.

While maximizing NILS at selected sampling locations in the patternenhances the budget/tolerance for exposure variation, commonly referredto as the exposure latitude EL, it may not help to increase thebudget/tolerance for focus variations, commonly referred to as the depthof focus (DOF). Furthermore, results obtained with this approachgenerally may suffer at low k1, where pure image calculations deviatesubstantially from printed substrate results.

SUMMARY

It is desirable to optimize the illumination conditions of alithographic apparatus to print features with greater precision.

In an embodiment of the invention, there is provided a method forconfiguring an illumination source of a lithographic apparatus, themethod including dividing the illumination source into pixel groups,each pixel group including one or more illumination source points;selecting an illumination shape to expose a pattern, the illuminationshape formed with at least one pixel group; iteratively calculating alithographic metric as a result of a change of state of a pixel group inthe illumination source, the change of the state of the pixel groupcreating a modified illumination shape; and adjusting the illuminationshape based on the iterative results of calculations.

In an embodiment of the invention, there is provided a computer producthaving machine executable instructions, the instructions beingexecutable by a machine to perform a method for configuring anillumination source of a lithographic apparatus, the method includingdividing the illumination source into pixel groups, each pixel groupincluding one or more illumination source points; selecting anillumination shape to expose a pattern, the illumination shape formedwith at least one pixel group; iteratively calculating a lithographicmetric as a result of a change of state of a pixel group in theillumination source, the change of the state of the pixel group creatinga modified illumination shape; and adjusting the illumination shapebased on the iterative results of calculations.

In an embodiment of the invention, there is provided a lithographicapparatus including an illumination system configured to condition abeam of radiation; a substrate table configured to hold a substrate; aprojection system configured to project a beam of radiation patterned bya patterning device onto a surface of the substrate; and a processor toperform a method for configuring an illumination source of alithographic apparatus, the method including dividing the illuminationsource into pixel groups, each pixel group including one or moreillumination source points; selecting an illumination shape to expose apattern, the illumination shape formed with at least one pixel group;iteratively calculating a lithographic metric as a result of a change ofstate of a pixel group in the illumination source, the change of thestate of the pixel group creating a modified illumination shape; andadjusting the illumination shape based on the iterative results ofcalculations

In an embodiment, there is provided a method for configuring anillumination source of a lithographic apparatus. The method includesdividing the illumination source into pixel groups, each pixel groupincluding one or more illumination source points; selecting anillumination shape to expose a pattern, the illumination shape formedwith at least one pixel group; changing a state of a pixel group in theillumination source to create a modified illumination shape; calculatinga variation of an attribute of an image of the pattern as a result ofthe change of state of the pixel group; and adjusting the initialillumination shape based on the results of the calculating.

In another embodiment of the invention, there is provided a method forconfiguring an illumination source of a lithographic apparatus, themethod including dividing the illumination source into pixel groups,each pixel group including one or more illumination source points;selecting an illumination shape to expose a pattern; calculating aplurality of responses for a plurality of changes in the illuminationshape, each of the plurality of changes effected by a change of state ofa pixel group; and adjusting the illumination shape based on theplurality of responses.

In yet another embodiment of the invention, there is provided a computerproduct having machine executable instructions, the instructions beingexecutable by a machine to perform a method for configuring anillumination source of a lithographic apparatus, the method includingdividing the illumination source into pixel groups, each pixel groupincluding one or more illumination source points; selecting anillumination shape to expose a pattern, the illumination shape formedwith at least one pixel group; changing a state of a pixel group in theillumination source to create a modified illumination shape; calculatinga variation of an attribute of an image of the pattern as a result ofthe change of state of the pixel group; and adjusting the initialillumination shape based on the results of the calculating.

In an embodiment of the invention, there is provided a lithographicapparatus including an illumination system configured to condition abeam of radiation; a substrate table configured to hold a substrate; aprojection system configured to project a beam of radiation patterned bya patterning device onto a surface of the substrate; and a processor toperform a method for configuring an illumination source of alithographic apparatus, the method including dividing the illuminationsource into pixel groups, each pixel group including one or moreillumination source points; selecting an illumination shape to expose apattern, the illumination shape formed with at least one pixel group;changing a state of a pixel group in the illumination source to create amodified illumination shape; calculating a variation of an attribute ofan image of the pattern as a result of the change of state of the pixelgroup; and adjusting the initial illumination shape based on the resultsof the calculating.

In an embodiment, there is provided a lithographic apparatus includingan illumination system configured to condition a beam of radiation; asubstrate table configured to hold a substrate; a projection systemconfigured to project a beam of radiation patterned by a patterningdevice onto a surface of the substrate; and a processor configured tocontrol an illumination source in a pupil plane of the illuminationsystem, the processor configured to divide the illumination source intopixel groups, each pixel group including one or more illumination sourcepoints; select an illumination shape to expose a pattern, theillumination shape formed with at least one pixel group; iterativelycalculate a lithographic metric as a result of a change of state of apixel group in the illumination source, the change of the state of thepixel group creating a modified illumination shape; and adjusting theillumination shape based on the iterative results of calculations.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 schematically depicts a lithographic apparatus in accordance withan embodiment of the invention;

FIG. 2 shows an exemplary flowchart for configuring the illuminationsource in accordance with an embodiment of the invention;

FIG. 3 is a schematic representation of a grid of illumination sourcepoints that may be used to model the illumination beam in the pupilplane of the illumination system;

FIG. 4 shows the spatial position of each pixel group in a quadrant ofthe grid shown in FIG. 3;

FIG. 5 a shows the spatial position of a pixel group in the grid shownin FIG. 3.

FIG. 5 b shows the spatial position of a pixel group different from theone shown in FIG. 5 a, in the grid shown in FIG. 3;

FIG. 6 shows an exemplary flowchart for configuring the illuminationsource in accordance with an embodiment of the invention;

FIG. 7 a shows a patterning device pattern that may be used to configurean illumination source in accordance with an embodiment of theinvention;

FIG. 7 b shows an initial illumination source that may be used to printthe pattern of FIG. 7 a;

FIG. 7 c shows the variation of CD uniformity as a function of pixelgroup position in the illumination source of FIG. 7 b;

FIG. 7 d shows an illumination source that has been optimized with themethod of FIG. 2;

FIG. 7 e shows the variation of CD uniformity as a function of pixelgroup position in the illumination source of FIG. 7 d;

FIG. 7 f shows an illumination source that has been optimized with themethod of FIG. 2;

FIGS. 8 a-b show the illumination source of FIG. 7 f as viewed by asimulation software and after a 0.05 Gaussian treatment;

FIGS. 9 a-b show the critical dimension uniformity CDU of, respectively,the vertical and horizontal gaps of the pattern shown in FIG. 7 aobtained with (a) an illumination setting configured in accordance withthe method of FIG. 2 and (b) an illumination setting configured with aconventional aerial image simulation;

FIG. 9 c shows the illumination source configured with a conventionalaerial image simulation;

FIG. 10 shows an exemplary flowchart for configuring the illuminationsource in accordance with an embodiment of the invention;

FIG. 11 a shows the variation of the critical dimension uniformity CDUof a pattern of holes as a function of pitch obtained with (a) anillumination source configured in accordance with the method of FIG. 10and (b) an illumination source configured with aparametrically-constrained source shape optimization;

FIGS. 11 b-c show, respectively, the illumination shape optimized inaccordance with the method of FIG. 10 and the second illumination sourceoptimized with a parametrically-constrained source shape optimization;

FIG. 12 shows a complex pattern of holes that represents a difficultcurrent imaging problem; a solution for imaging this pattern may befound by configuring an illumination source in accordance with anembodiment of the invention;

FIG. 13 a shows the variation of exposure latitude as a function ofdepth of focus for a pattern of 75 nm isolated trenches and for (a) asource optimized in accordance with the method of FIG. 2 using isofocalcompensation, (b) a circular illumination source (σ_(r)=0.6) and (c) anannular illumination source (σ_(c)=0.6; σ_(r)=0.9);

FIG. 13 b shows the illumination source optimized in accordance with themethod of FIG. 2 using isofocal compensation, in accordance with anembodiment of the invention; and

FIG. 13 c shows a pattern of 75 nm isolated trenches.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the invention. The apparatus includes an illuminationsystem (illuminator) IL adapted to condition a beam B of radiation(e.g., UV radiation) and a support structure (e.g., a mask table) MTconfigured to hold a patterning device (e.g., a mask) MA and connectedto a first positioning device PM configured to accurately position thepatterning device with respect to the projection system PS. Theapparatus also includes a substrate table (e.g., a wafer table) WTconfigured to hold a substrate (e.g., a resist-coated wafer) W andconnected to a second positioning device PW configured to accuratelyposition the substrate with respect to the projection system PS. Theapparatus also includes a projection system (e.g., a refractiveprojection lens) PS adapted to image a pattern imparted to the beam B bythe patterning device MA onto a target portion C (e.g., including one ormore dies) of the substrate W.

As depicted here, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to below).

The illuminator IL receives a beam of radiation from a radiation sourceSO. The source and the lithographic apparatus may be separate entities,for example when the source is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam is passed from the source SO to the illuminator ILwith the aid of a beam delivery system BD, including, for example,suitable directing mirrors and/or a beam expander. In other cases, thesource may be an integral part of the apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, may be referred to as aradiation system.

The projection system PS may include a diaphragm with an adjustableclear aperture used to set the numerical aperture of the projectionsystem PS at substrate level to a selected value. The selectablenumerical aperture, or, in the case of a fixed clear aperture, the fixednumerical aperture, will be referred to as NA_(PS). At patterning device(e.g., mask) level, a corresponding angular capture range within whichthe projection system PS is capable of receiving rays of the beam ofradiation is given by the object-side numerical aperture of theprojection system PS, referred to as NA_(PSOB). The object-sidenumerical aperture of the projection system PS is denoted by NA_(PSOB).Projection systems in optical lithography are commonly embodied asreduction projection systems with a reduction ratio M of, for example,5× or 4×. A numerical aperture NA_(PSOB) is related to NA_(PS) throughthe reduction ratio M by NA_(PSOB)=NA_(PS)/M.

The beam of radiation B provided by the illumination system IL to thepatterning device MA includes a plurality of radiation rays that impingeonto the patterning device MA with a plurality of angles of incidence.These angles of incidence are defined with respect to the Z axis inFIG. 1. These rays can therefore be characterized by an illuminationnumerical aperture NA_(IL), which is defined by NA_(IL)=sin(angle ofincidence), where the index of refraction of the space upstream of themask is assumed to be 1. However, instead of characterizing theillumination radiation rays by the numerical aperture NA_(IL), the raysmay alternatively be characterized by the radial positions of thecorresponding points traversed by these rays in a pupil of theillumination system. The radial position is linearly related to NA_(IL),and it is common practice to define a corresponding normalized radialposition σ in a pupil of the illumination system by σ=NA_(IL)/NA_(PSOB).

In addition to an integrator IN and a condensor CO, the illuminationsystem typically includes an adjusting device AM configured to set anouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in the pupil of theillumination system. In view of the normalization, when σ-outer=1,radiation rays traversing the edge of the illumination pupil can becaptured (in the absence of diffraction by the patterning device (e.g.,mask) MA) by the projection system PS, because then NA_(IL)=NA_(PSOB).

The beam of radiation B is incident on the patterning device MA, whichis held on the support structure MT. Having traversed the patterningdevice MA, the beam of radiation B passes through the projection systemPS, which focuses the beam onto a target portion C of the substrate W.With the aid of the second positioning device PW and position sensor IF(e.g., an interferometric device), the substrate table WT can be movedaccurately, e.g., so as to position different target portions C in thepath of the beam B. Similarly, the first positioning device PM andanother position sensor (which is not explicitly depicted in FIG. 2) canbe used to accurately position the patterning device MA with respect tothe path of the beam B, e.g., after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the support structureMT and substrate table WT will be realized with the aid of a long-strokemodule (coarse positioning) and a short-stroke module (finepositioning), which form part of one or both of the positioning devicesPM and PW. However, in the case of a stepper (as opposed to a scanner)the support structure MT may be connected to a short stroke actuatoronly, or may be fixed. Patterning device MA and substrate W may bealigned using patterning device alignment marks M1, M2 and substratealignment marks P1, P2.

The depicted apparatus may be used in the following preferred modes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to thebeam of radiation is projected onto a target portion C at once (i.e., asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the beam of radiationis projected onto a target portion C (i.e., a single dynamic exposure).The velocity and direction of the substrate table WT relative to thesupport structure MT is determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the projectionbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizes aprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations of the above described modes of use orentirely different modes of use may also be employed.

In accordance with at least one embodiment of the invention, it isproposed to configure and optimize an illumination source and the shapeof the source in view of an intended use of a selected patterning-devicepattern for lithographically printing an IC pattern. For example, FIG. 2shows a method for configuring the illumination source of a lithographicapparatus in accordance with an embodiment of the invention.

The method starts at block 201 and proceeds to block 202 where theillumination source of the illumination system IL is divided into aplurality of pixel groups or segments. Each pixel group or segment ofthe illumination source includes one or more illumination source points.

FIG. 3 shows a schematic representation of a grid 300 of illuminationsource points 305 that may be used to model the illumination radiationbeam B in the pupil plane of the illumination system IL. The grid 300 iscentered on the optical axis of the lithographic apparatus. In FIG. 3,this optical axis is parallel to the Z axis. Decomposition of theillumination source into a plurality of individual source points is usedin Abbe imaging to calculate the image of a pattern onto the substrate.Under Abbe's formulation, the intensity on the substrate of an image ofthe patterning device (e.g., mask) is calculated by dividing theillumination source into a number of individual source points. Eachindividual illumination source point that is part of the illuminationbeam illuminates the patterning device. The total intensity on thesubstrate is calculated by summing the intensity resulting from eachindividual source point.

The illumination source points 305 form a grid that spatially covers theentire cross-section of a clear aperture 310 in the pupil plane of theillumination system IL. The clear aperture 310 in the pupil plane of theillumination system IL has a normalized radius value of σ_(r)=1. Thesource points 305 located inside the numerical aperture will have theirzero^(th) diffraction order captured by the projection system PS. Thissituation corresponds to σ_(r)≦1. The physical location of each sourcepoint 305 relative to the full illuminator aperture may be varieddepending on the degree of accuracy desired. For example, the gridspacing may be approximately 0.01. In the embodiment represented in FIG.3, the grid 300 is constructed with a 51*51 square source point grid.Thus, in this implementation, the normalized diameter of the grid 300includes 51 illumination source points. It will be appreciated that thenumber of illumination source points may vary in other embodiments ofthe illumination.

In accordance with block 202 of FIG. 2, grid 300 is divided into aplurality of pixel groups or segments that include one or moreillumination source points 305. Each pixel group is defined by its polarcoordinates (θ^(r), θ). For each pixel group, radius σ^(r) defines thedistance from the center of the grid 300, which corresponds to theoptical axis of the lithographic system, to the center of the pixelgroup. Angle θ corresponds to the angular position of the pixel grouprelative to the X axis. In FIG. 3, the X axis is substantially parallelto one of the main directions of the patterns that are being imaged. Forexample, the X axis may be parallel to the vertical or horizontaldirection of the patterns. It will be appreciated that the orientationof the X axis may be different in other embodiments of the invention.

The total number of pixel groups that are considered (see block 205) mayvary depending on the geometry of the pattern(s) that is or are beingconsidered in the configuration or illumination process. In oneimplementation of the method of FIG. 2, two different symmetry classesmay be considered to define the total number of pixel groups. Forexample, for simple pattern(s) with C4 symmetry, fewer pixel groups maybe needed to perform the illumination configuration or optimizationbecause the groups also have C4 symmetry and more pixels in each groupthan groups with lower symmetry. An example of such a pattern includes apattern of horizontal and vertical lines or a square array of holes.These patterns are not changed by a 90° rotation. Therefore, theilluminator or illumination system will be symmetric about the X and Yaxes and the diagonals. In this configuration, a pixel group in oneoctant can be symmetrically reproduced in the other octants during theconfiguration of the illumination conditions. Alternatively, forpattern(s) with lower symmetry, it may be desirable to use additionalpixel groups to perform the illumination configuration, as each groupalso should have lower symmetry. An example of a pattern including twoplanes of symmetry is a pattern of vertical line, horizontal line orbrickwall. A single V-line, brickwall, rectangular hole grid and chevronpattern have lower symmetry and a pixel group in one octant can bereflected in the X and Y axes but not the diagonals.

Referring to FIG. 4, this figure shows the spatial position of eachpixel group in the upper right quadrant of grid 300 in accordance withat least one embodiment of the invention. This quadrant includes 117pixel groups. The positions of the pixel groups in the remainingquadrants of the grid 300 can be obtained by symmetry with respect tothe X and Y axes. The map assignment for the pixel groups of FIG. 4corresponds to a two-symmetry plane configuration. In thisconfiguration, each pixel group is symmetrically mapped with respect tothe X and Y axes. For reference, the coordinate position of each pixelgroup is shown in Table 1. As each pixel group may include more than oneillumination source point, the radial and angular position (σ_(r), θ) ofeach pixel group includes a maximum and a minimum value.

TABLE 1 Pixel σ_(r) σ_(r) θ θ Group min max min max 1 0 0.1 0 90 2 0.10.15 0 90 3 0.15 0.2 0 90 4 0.2 0.25 0 22.5 5 0.2 0.25 22.5 45 6 0.20.25 45 67.5 7 0.2 0.25 67.5 90 8 0.25 0.3 0 22.5 9 0.25 0.3 22.5 45 100.25 0.3 45 67.5 11 0.25 0.3 67.5 90 12 0.3 0.35 0 22.5 13 0.3 0.35 22.545 14 0.3 0.35 45 67.5 15 0.3 0.35 67.5 90 16 0.35 0.4 0 22.5 17 0.350.4 22.5 45 18 0.35 0.4 45 67.5 19 0.35 0.4 67.5 90 20 0.4 0.44 0 22.521 0.4 0.44 22.5 45 22 0.4 0.44 45 67.5 23 0.4 0.44 67.5 90 24 0.44 0.480 22.5 25 0.44 0.48 22.5 45 26 0.44 0.48 45 67.5 27 0.44 0.48 67.5 90 280.48 0.52 0 22.5 29 0.48 0.52 22.5 45 30 0.48 0.52 45 67.5 31 0.48 0.5267.5 90 32 0.52 0.56 0 15 33 0.52 0.56 15 30 34 0.52 0.56 30 45 35 0.520.56 45 60 36 0.52 0.56 60 75 37 0.52 0.56 75 90 38 0.56 0.6 0 15 390.56 0.6 15 30 40 0.56 0.6 30 45 41 0.56 0.6 45 60 42 0.56 0.6 60 75 430.56 0.6 75 90 44 0.6 0.64 0 15 45 0.6 0.64 15 30 46 0.6 0.64 30 45 470.6 0.64 45 60 48 0.6 0.64 60 75 49 0.6 0.64 75 90 50 0.64 0.68 0 15 510.64 0.68 15 30 52 0.64 0.68 30 45 53 0.64 0.68 45 60 54 0.64 0.68 60 7555 0.64 0.68 75 90 56 0.68 0.72 0 15 57 0.68 0.72 15 30 58 0.68 0.72 3045 59 0.68 0.72 45 60 60 0.68 0.72 60 75 61 0.68 0.72 75 90 62 0.72 0.760 11.25 63 0.72 0.76 11.25 22.5 64 0.72 0.76 22.5 33.75 65 0.72 0.7633.75 45 66 0.72 0.76 45 56.25 67 0.72 0.76 56.25 67.5 68 0.72 0.76 67.578.75 69 0.72 0.76 78.75 90 70 0.76 0.8 0 11.25 71 0.76 0.8 11.25 22.572 0.76 0.8 22.5 33.75 73 0.76 0.8 33.75 45 74 0.76 0.8 45 56.25 75 0.760.8 56.25 67.5 76 0.76 0.8 67.5 78.75 77 0.76 0.8 78.75 90 78 0.8 0.84 011.25 79 0.8 0.84 11.25 22.5 80 0.8 0.84 22.5 33.75 81 0.8 0.84 33.75 4582 0.8 0.84 45 56.25 83 0.8 0.84 56.25 67.5 84 0.8 0.84 67.5 78.75 850.8 0.84 78.75 90 86 0.84 0.88 0 11.25 87 0.84 0.88 11.25 22.5 88 0.840.88 22.5 33.75 89 0.84 0.88 33.75 45 90 0.84 0.88 45 56.25 91 0.84 0.8856.25 67.5 92 0.84 0.88 67.5 78.75 93 0.84 0.88 78.75 90 94 0.88 0.92 011.25 95 0.88 0.92 11.25 22.5 96 0.88 0.92 22.5 33.75 97 0.88 0.92 33.7545 98 0.88 0.92 45 56.25 99 0.88 0.92 56.25 67.5 100 0.88 0.92 67.578.75 101 0.88 0.92 78.75 90 102 0.92 0.95 0 11.25 103 0.92 0.95 11.2522.5 104 0.92 0.95 22.5 33.75 105 0.92 0.95 33.75 45 106 0.92 0.95 4556.25 107 0.92 0.95 56.25 67.5 108 0.92 0.95 67.5 78.75 109 0.92 0.9578.75 90 110 0.95 0.98 0 11.25 111 0.95 0.98 11.25 22.5 112 0.95 0.9822.5 33.75 113 0.95 0.98 33.75 45 114 0.95 0.98 45 56.25 115 0.95 0.9856.25 67.5 116 0.95 0.98 67.5 78.75 117 0.95 0.98 78.75 90Table 2 shows the spatial position of a set of pixel groupscorresponding to a four-symmetry plane configuration. In thisconfiguration, each pixel group is symmetrically mapped with respect tothe X and Y axes and the diagonal of the quadrant.

TABLE 2 Pixel σ_(r) σ_(r) θ θ Group min max min max 1 0 0.1 0 90 2 0.10.15 0 90 3 0.15 0.2 0 90 4 0.2 0.25 0 22.5 5 0.2 0.25 22.5 45 6 0.250.3 0 22.5 7 0.25 0.3 22.5 45 8 0.3 0.35 0 22.5 9 0.3 0.35 22.5 45 100.35 0.4 0 22.5 11 0.35 0.4 22.5 45 12 0.4 0.44 0 22.5 13 0.4 0.44 22.545 14 0.44 0.48 0 22.5 15 0.44 0.48 22.5 45 16 0.48 0.52 0 22.5 17 0.480.52 22.5 45 18 0.52 0.56 0 15 19 0.52 0.56 15 30 20 0.52 0.56 30 45 210.56 0.6 0 15 22 0.56 0.6 15 30 23 0.56 0.6 30 45 24 0.6 0.64 0 15 250.6 0.64 15 30 26 0.6 0.64 30 45 27 0.64 0.68 0 15 28 0.64 0.68 15 30 290.64 0.68 30 45 30 0.68 0.72 0 15 31 0.68 0.72 15 30 32 0.68 0.72 30 4533 0.72 0.76 0 11.25 34 0.72 0.76 11.25 22.5 35 0.72 0.76 22.5 33.75 360.72 0.76 33.75 45 37 0.76 0.8 0 11.25 38 0.76 0.8 11.25 22.5 39 0.760.8 22.5 33.75 40 0.76 0.8 33.75 45 41 0.8 0.84 0 11.25 42 0.8 0.8411.25 22.5 43 0.8 0.84 22.5 33.75 44 0.8 0.84 33.75 45 45 0.84 0.88 011.25 46 0.84 0.88 11.25 22.5 47 0.84 0.88 22.5 33.75 48 0.84 0.88 33.7545 49 0.88 0.92 0 11.25 50 0.88 0.92 11.25 22.5 51 0.88 0.92 22.5 33.7552 0.88 0.92 33.75 45 53 0.92 0.95 0 11.25 54 0.92 0.95 11.25 22.5 550.92 0.95 22.5 33.75 56 0.92 0.95 33.75 45 57 0.95 0.98 0 11.25 58 0.950.98 11.25 22.5 59 0.95 0.98 22.5 33.75 60 0.95 0.98 33.75 45

FIGS. 5 a-b show respectively the positions in grid 300 of the first andthe seventy-fifth pixel groups of Table 1. FIG. 5 a shows grid 300without the first pixel group. As can be seen in FIG. 5 a, the firstpixel group includes the source points 305 that have a normalized radiusσ_(r) in the 0-0.1 range and an angular position in the 0-90° range(with respect to the quadrant shown in FIG. 4). As can be seen in FIG. 5b, the seventy-fifth pixel group includes source points that have anormalized radius σr in the 0.76-0.8 range and an angular position inthe 56.25-67.5° range (with respect to the quadrant shown in FIG. 4).The seventy-fifth pixel group includes three illumination source pointsin each quadrant.

In Tables 1 and 2, pixel groups with a normalized σ_(r) radius greaterthan 0.98 have been excluded because typical modern illuminators cutoffat σ_(r)≈0.98.

Referring back to FIG. 2, after dividing the illumination source intopixel groups, the method then proceeds to block 203 where an initialillumination setting (e.g., illumination shape or illumination mode) isselected. The initial illumination shape includes one or more pixelgroups and is chosen for its aptitude to print the features of interestin the patterning device (e.g., lines, holes, or brickwalls). Theinitial illumination setting may be determined either viaexperimentation or simulation, as will be appreciated by one of ordinaryskill in the art. In at least one implementation, the initialillumination setting is configured to print the pattern with sufficientprocess latitude (e.g., exposure latitude and depth of focus).

The method 200 then proceeds to block 204 where the state of a pixelgroup is changed in the illumination source to create a modifiedillumination shape. The state of the pixel group may be changed byadding the pixel group to the initial illumination shape. Alternatively,the state of the pixel group may be changed by removing the pixel groupfrom the initial illumination shape. When a pixel group is removed fromthe illumination shape, it does not contribute to the illumination beam.When a pixel group is added in the initial illumination shape, itcontributes to the illumination beam.

Then, after changing the state of a pixel group, the method proceeds toblock 205 where a lithographic metric is calculated or estimated (e.g.,a characteristic of an image of the pattern) as a result of the changeof state in the pixel group. The lithographic metric is a value functionthat is selected to estimate and/or gauge the performance of themodified illumination shape when it illuminates the pattern. Moregenerally, the lithographic metric may also be termed a lithographicresponse. The lithographic metric may be, for example, a criticaldimension uniformity of the printed pattern, a dimension of the processwindow, mask error enhancement factor (MEEF), maximum NILS or maximumNILS in defocus. It will be appreciated that additional lithographicmetrics or responses may be used in an embodiment of the invention.Then, the method proceeds to block 206 where the initial illuminationshape is adjusted based on a result of the calculation. After adjustingthe initial illumination shape based on the result of the calculating,the method proceeds to block 207 where a determination is made as towhether a state of another pixel group should be changed. If the resultof the inquiry is positive, the method proceeds back to block 204 andcontinues through block 206. If the result of the inquiry is negative,the method ends at block 208. As will be explained in more detail below,the operations of blocks 204, 205 and 206 may be iteratively executedfor a plurality of pixel groups.

Referring to FIG. 6, this figure shows a method for executing theoperations of blocks 204, 205, 206 and 207 of FIG. 2, in accordance withan embodiment of the invention. In FIG. 6, the method 600 starts atblock 601 and proceeds to block 605 where a first pixel group isinterrogated or selected. The method then proceeds to block 610 where adetermination is made as to whether the first interrogated pixel groupis “on” or “off”. When a pixel group is “on” (or active), it is part ofthe initial illumination shape and provides at least a portion of theillumination beam B. By contrast, when a pixel group is “off” (orinactive), it does not contribute to the initial illumination shape and,therefore, does not illuminate the patterning device. In the method ofFIG. 6, if the first pixel is “on”, it is turned “off” (block 615 a),and if the first pixel is “off”, it is turned “on” (block 615 b). In sodoing, a modified illumination shape may be obtained.

After flipping the first pixel group, the method then proceeds to block620 where a photolithographic metric or response is calculated for themodified illumination shape or setting. In an embodiment, thephotolithographic metric or response may be a critical dimensionuniformity of the printed pattern, a dimension of the process window,mask error enhancement factor (MEEF), maximum NILS or maximum NILS indefocus. It will be appreciated that additional lithographic metrics orresponses may be used in an embodiment of the invention. Calculation ofthe photolithographic metric or response may be done usingphotolithographic simulations.

Calculation of the photolithographic metric may involve firstcalculating one or more images of the pattern illuminated by themodified illumination shape. Calculation of the image(s) may be doneusing an aerial image model or a resist model. After calculating theimage(s) of the pattern, an attribute of the calculated image(s) may beevaluated to estimate and/or gauge the performance of the modifiedillumination shape. In this example, the attribute corresponds to thelithographic metric or response. The photolithographic simulationscalculate the photolithographic metric or response for the pattern whenit is illuminated by the modified initial illumination shape. Thephotolithographic simulations may be performed using various models.

In at least one implementation, calculation of the photolithographicmetric or response may be performed with a resist model. The resistmodel may take into account the resist baking and the resist developingto calculate the image of the pattern in the resist. The resist modelmay also take into account the non-planar topography at the surface ofthe substrate and the vector effects. The vector effects refer to thefact that an electromagnetic wave propagates obliquely when a highnumerical aperture is used. Alternatively, a lumped parameter model or avariable threshold resist model may also be used in at least oneembodiment of the invention to calculate the photolithographic metric orresponse. A calibrated model is a model that has been matched toexperimental data.

In another implementation, an aerial image model may be used tocalculate the photolithographic metric or response. Calculation of theaerial image may be done either in the scalar or vector form of theFourier optics. The quality of the aerial image may be determined byusing a contrast or normalized aerial image log-slope (NILS) metric(normalized to the feature size). This value corresponds to the slope ofthe image intensity (or aerial image).

Relevant parameters to perform the photolithographic simulations mayinclude the distance from the focal plane of the Gaussian image plane,meaning the distance to the plane where the best plane of focus exists,as determined by geometrical ray optics, or the center wavelength of thequasi-monochromatic radiation source. The parameters may also include ameasure of degree of spatial partial coherence of the illuminationsystem, the numerical aperture of the projection system exposing thesubstrate and the aberrations of the optical system. In practice, thephotolithographic simulations may be carried out with the aid of acommercially available simulator such as Prolith™, Solid-C™ or the like.

In an embodiment of the invention, the photolithographic metric may be apredicted critical dimension uniformity (CDU) of the printed patternwherein the predicted value of the metric is calibrated to match ameasured CDU of the printed pattern on the substrate. Using thisembodiment may be beneficial in a lithographic manufacturing processusing a patterning device (e.g. mask) wherein the patterning device(e.g. mask) has an error. Due to the patterning device error, theprinted pattern on the substrate has a CDU that is different from thepredicted CDU, which was calculated for a patterning device (e.g. mask)having no error. By matching the predicted CDU to the measured CDU, theeffect of the patterning device error may be corrected using theflowchart of FIG. 2. This embodiment according to the invention mayfurther be beneficial when the patterning device (e.g. mask) ismanufactured within the specifications of the patterning device designbut where the OPC model used to calculate OPC features on the patterningdevice (e.g. mask) was incorrect or not sufficiently accurate. Anincorrect or not sufficiently correct OPC model may for example beobtained when scaling a pattern design containing OPC to a smallertechnology node. In such a scaling process, both pattern features andOPC features are scaled to a smaller feature size. The effect of thesmaller OPC features may be different than expected due to anon-linearity of the effect. By matching the predicted CDU to themeasured CDU, the effect of the incorrect or inaccurate OPC model may becorrected using the flowchart of FIG. 2. Furthermore, this embodimentaccording to the invention may be beneficially used for correctingdifferences in a predicted CDU and a measured CDU caused by otherprocess parameters that deviate from their expected setting, such asdeviations in the illumination system or projection system.

Additional information regarding the calculations of an image of apatterning device (e.g., mask) pattern using photolithographicsimulations may be gleaned, for example, in U.S. Pat. No. 6,839,125issued on Dec. 16, 2004, entitled “Method for Optimizing an IlluminationSource Using Full Resist Simulation and Process Window Metric” and U.S.Pat. No. 7,016,017, issued on Mar. 1, 2006, entitled “LithographicApparatus and Method for Optimizing an Illumination Source UsingIsofocal Compensation.” The contents of these two applications areincorporated herein in their entirety by reference.

The photolithographic metric or response (e.g., the imaged pattern) maybe calculated over an assumed budget of process parameters (e.g., focus,dose and patterning device (e.g., mask) variation). For example, in atleast one implementation, the lithographic metric or response may becalculated by adding in quadrature the variations of the metric orresponse due to each of the process parameters. In this example, themetric or response corresponds to the total variation of the metric forthe assumed budget of process parameters.

After calculating the lithographic metric or response, the method thenproceeds to block 630 where a determination is made as to whether thevalue of the metric or response has been improved due to the change(i.e., pixel group flipping) of the initial illumination shape. Forexample, if the metric or response is a critical dimension uniformity ofthe pattern, the determination of block 630 is whether the CDnon-uniformity has been reduced or increased as a result of the changein the initial illumination shape. As another example, if the metric orresponse is a dimension of the process window, the determination ofblock 630 is whether the dimension of the process window has beenreduced or increased as a result of the change in the initialillumination shape. If the result of the inquiry is positive, the changemade to the first pixel group is kept (block 635 a). Alternatively, ifthe result of the inquiry is negative, the change made to the firstpixel group is ignored, and the pixel group is flipped back to itsoriginal state (i.e., if the pixel group was originally “on”, it remains“on”, and if the pixel group was originally “off”, it remains “off”.)

Then, after assessing the influence of the first pixel group on thequality of the imaged pattern by using the lithographic metric orresponse, a determination is made as to whether all the pixel groupshave been interrogated (block 640). If the result of the inquiry isnegative, the method 600 proceeds back to block 605 where a second pixelgroup is interrogated or selected. Then, the method proceeds again fromblock 610 to block 640 in a similar manner as before. This process isiteratively executed until all the selected pixel groups have beeninterrogated. If the result of the inquiry is negative, the method 600ends at block 645.

Blocks 601-645, shown in FIG. 6, represent one method that may be usedto calculate at least one response obtained for at least one change inthe initial illumination shape. In the embodiment of FIG. 6, theresponse may correspond to the variation of a critical dimension of thepattern. As will be appreciated, calculations of the series of responsesin FIG. 2 provide an optimized illumination setting which optimizes(e.g., maximizes or minimizes) a particular attribute (e.g., CDvariations or dimension of the process window) of the photolithographicresponse (e.g., image pattern).

Referring back to FIG. 2, once the modified or optimized illuminationshape has been determined, the process for adjusting (e.g., optimizingor configuring) the illumination source shown in FIG. 2 may beiteratively repeated. For example, after calculating the modifiedillumination shape at block 206, the method may proceed back to block203, where the modified illumination setting may act as a new initialillumination setting. In this embodiment, the method proceeds from block203 to block 206 in a similar manner as before so that a new modified oroptimized illumination setting may be calculated. This process may berepeated if desired. Then, the method ends at block 208.

All the pixel groups of the illumination source or portion thereof maybe interrogated. For example, only the pixel groups having a normalizedradial and/or angular value greater than a predetermined value may beinterrogated. As another example, only the pixel groups that are part ofthe illumination beam or initial illumination shape may be interrogated.In addition, it will be appreciated that various metrics, responses orattributes may be used in determining the modified illumination setting.Selection of the metric, response or attribute may be based on thegeometry of the pattern that is being imaged.

In one implementation, the illumination source may be optimized usinggroup pixel flipping to image the DRAM structure 700 shown in FIG. 7 a.The structure 700, commonly referred to as a herringbone or chevronstructure, includes a plurality of rectangles 701 that are inclinedrelative to each other at a particular angle. Each rectangle 701 isdefined by its length 702, width 703 and bridge width 704. FIG. 7 a alsoshows the horizontal gap 705 and the vertical gap 706 between twoadjacent rectangles.

FIG. 7 b shows an initial illumination setting 710 that may be used toimage and print the structure 700 on the substrate. The initial shapehas an annular shape having a normalized inner radius 711 (σ_(c)) ofabout 0.72 and a normalized outer radius 712 (σ_(r)) of about 0.98. Thisinitial illumination shape may be initially determined based onexperimental data or preliminary simulations.

In accordance with the methods of FIGS. 2 and 6, the illumination shape710 is modified to configure or optimize the printing of pattern 700. Inthis configuration process, only the pixel groups having a normalizedradius greater than 0.72 are being considered. These pixel groupscorrespond to pixel group nos. 61-117 in Table 1. In accordance with themethods of FIGS. 2 and 6, each of these pixel groups is interrogated anda determination is made as to whether the flipping of each pixel groupreduces or increases the variation of the attribute. Calculation of theimage of the pattern is done with a calibrated lump parameter model.Calculations are done with a 1.2 numerical aperture (corresponding tok1=0.31) and a TE polarized illumination beam. The horizontal half pitchbetween two adjacent rectangles is 50 nm.

In this embodiment, the metric that is being considered to gauge theinfluence of the pixel groups on the quality of the imaged pattern isthe combination of the critical dimension uniformity of the width (CDwidth—703 set to 50 nm) and the critical dimension uniformity of thelength (CD length—702) of the rectangle 701. The metric that is beingused to determine the influence of the pixel groups is defined inequation (2):CDU total=(2 CDU width+CDU length)/3  (2)The metric CDU (critical dimension uniformity) total in equation (2)includes the weighted sum of the critical dimension (CD) variations ofthe width and the length of the pattern 701 as a function of a specifiederror budget.

CDU width and CDU length are determined by calculating the variations ofCD width and CD length of the pattern 701. Each of the variations of CDwidth and CD length is calculated over an assumed budget of dose, focusand mask error with the following equation:CD=√{square root over (CD_(Rfoc) ²+CD_(Rdose) ²+CD_(Rglobalmask)²)}  (3)where CD_(Rfoc), CD_(Rdose), and CD_(Rglobalmask) correspond to the CDtotal variation induced by focus, dose, and mask variations,respectively, over an assumed budget. In one implementation, the assumedbudget includes a 0.2 μm defocus, a 2% variation dose and a 1 nm masksize error. In one embodiment, the critical dimension of the bridgingwidth at 5% underexposure may be used as an additional process error andcombined into the total CDU metric. The CD variation as defined inequation (3) substantially represents the full CD variation range of thepattern printed on the substrate and, as such, approximates the sixsigma statistical variation range. Thus, half of the value of the CDtotal variation (also termed as CD variation half range) substantiallyapproximates the normally used three sigma CD uniformity (CDU).

Referring to FIG. 7 c, this figure shows the influence of some pixelgroups on the critical dimension uniformity (CDU) that is calculatedwith equation (2). In FIG. 7 c, when a pixel group is retained, it isset to “1”. As can be seen in this figure, CDU is improved when pixelgroups 60 and 61 are removed. As a result, these pixel groups areremoved from the illumination setting. By contrast, CDU degrades whenpixel groups 62-66 are removed. As a result, these pixel groups arereturned to the illumination setting. This process continues for theremaining pixel groups 67-117. Each time a pixel group degrades CDU, itis removed from the illumination setting. Each time a pixel groupimproves CDU, it is kept in the illumination setting. As can be seen inFIG. 7 c, CDU is significantly improved as the pixel groups that degradeCDU are removed. In FIG. 7 c, the CDU of the imaged pattern is improvedby at least a factor of two with the new illumination setting.

FIG. 7 d shows the illumination setting after interrogating pixel groupnos. 60-117. As can be seen in FIG. 7 d, the initial source has beensignificantly altered by the optimization process of FIGS. 2 and 6.

It is also possible to repeat this optimization process and use themodified illumination shape as an initial illumination shape in order torefine the optimized illumination shape. FIG. 7 e shows the variation ofCDU as a function of the pixel groups that were retained after the firstpass (see FIG. 7 c). As shown in FIG. 7 e, some of the pixel groups(PG), which were originally retained after the first pass, are nowexcluded from the new refined illumination shape. The refinedillumination shape is shown in FIG. 7 f. Additional passes may beperformed to further refine the illumination shape. In practice, theseseparate passes can be combined into a single continuous process leadingto an optimal result.

As shown in FIG. 7 f, the refined illumination shape includes sharpedges that may be, in practice, difficult to create. In oneimplementation, the final illumination shape may further be refined withan appropriate software to soften its sharp edges to make the simulationmore representative of the actual implementation in a scanner. Forexample, the final illumination shape may be convoluted with a Gaussianfunction to soften the sharp edges. FIG. 8 a shows the optimum source asviewed by the simulation software Prolith™ in the pupil plane of theillumination system. FIG. 8 b shows the optimum illumination shape afterbeing convoluted with a Gaussian function. FIGS. 8 a-b also show theintensity of the illumination portions.

While only the pixel groups included in the initial annular illuminationshape (pixel groups with σr≧0.72) have been considered in FIGS. 7 a-f,it will be appreciated that additional pixel groups may also beconsidered to configure the illumination shape. For example, pixelgroups corresponding to σr<0.72 could also be considered in at least oneembodiment of the invention. In addition, it will be appreciated thatpixel groups may be interrogated/selected either randomly or with apredetermined order. In FIGS. 7 c and 7 e, pixel groups areinterrogated/selected as a function of an increase in radius.Alternatively, the pixel groups could be interrogated/selected as afunction of a decrease in radius. It will be appreciated that anycombination of the above could be used to interrogate/select the pixelgroups.

Referring now to FIGS. 9 a-b, these figures show the critical dimensionuniformity CDU of the vertical and horizontal gaps 706, 705,respectively, obtained with (a) an illumination shape configured inaccordance with the method of FIG. 2 (see FIG. 8 b) (referred to as“pixel flip” in FIG. 9 a) and (b) an illumination shape optimized usingconventional aerial image simulation (e.g., using a NILS metric andreferred to as “NILS” in FIG. 9 b). For reference, the illuminationshape optimized with a conventional aerial image simulation is shown inFIG. 9 c. CDU results are shown for an assumed budget of errors (masksize, focus and dose). The total CDU variation that results from thecombination of these errors is also shown in FIGS. 9 a-b. In FIGS. 9a-b, results are provided for the following assumed budget: 200 nm focusvariation, 2 nm mask variation and 2% dose variation. Calculations aredone with a 0.93 numerical aperture (corresponding to k1=0.35), a X+Ypolarized illumination beam and a 6% attenuated phase shift mask. Acalibrated lump parameter model is used in the pixel flippingoptimization and in the subsequent comparison of the two methods.

As can be seen in FIGS. 9 a-b, the critical dimension uniformity of boththe horizontal and vertical gaps 705,706 are significantly reduced whenthe pattern is illuminated with an illumination shape that is configuredin accordance with the method of FIG. 2. The CD control of therectangles 701 is much improved for each error (i.e., focus, mask anddose).

In one embodiment, the method of FIG. 2 is modified to configure oroptimize the illumination shape for a situation where several pitchesshould be optimally printed simultaneously. For example, referring toFIG. 10, this figure shows a method for configuring an illuminationsource in accordance with an embodiment of the invention. The method1000 of FIG. 10 is adapted to configure the illumination shape fordifferent pattern pitches. The method begins at block 1001 and proceedsto block 1005 and 1010 in the same way as in FIG. 2. That is, at block1005 the illumination source is divided into pixel groups in the pupilplane of the illumination system. Each pixel group includes one or moreillumination source points. At block 1010, an initial illumination shapeis selected. The initial illumination shape may be determined either viaexperimentation or simulation, as will be appreciated by one of ordinaryskill in the art. In at least one implementation, the initialillumination shape is configured to print the pattern with sufficientprocess latitude (e.g., exposure latitude and depth of focus) at variouspitches.

After selecting the initial illumination source, the method proceeds toblocks 1015 and 1020, where, respectively, a first pixel group isselected or interrogated and a first pitch is selected. At block 1015,the method proceeds in a similar manner as in FIG. 6. Specifically, adetermination is made as to whether the first interrogated pixel groupis “on” or “off”. When a pixel group is “on” (or active), it is part ofthe initial illumination shape and contributes to the illumination beam.By contrast, when a pixel group is “off” (or inactive), it is not partof the initial illumination shape and, therefore, does not illuminatethe patterning device. In the method of FIG. 10, if the interrogatedpixel is “on”, it is turned “off”, and if the interrogated pixel is“off”, it is turned “on”. In so doing, a modified illumination shape maybe obtained.

The method then proceeds to block 1025 where the patterning device bias,which may be required to print the pattern to the correct size, isdetermined. This variable bias is desirable because for an assumed doseor image threshold the correct bias is both source and pitch dependent.As will be appreciated, this bias may be calculated by photolithographicsimulations. Alternatively, instead of calculating a bias at block 1025,it is also possible to determine the desired dose to print the patternto the target size at a fixed bias. This dose is commonly known as thedose to size E1:1 and may also be determined using photolithographicsimulations. For an image calculation rather than E1:1, the appropriateimage threshold must be selected.

After determining the patterning device bias or the dose to size E1:1,the method then proceeds to block 1030 where a lithographic metric orresponse of the pattern for an assumed budget of errors is calculated.In one implementation, the metric or response that is considered togauge the performance of the illumination source is a critical dimensionuniformity of the pattern. It will be appreciated that alternativemetrics may also be used. Typically, at block 1030, the methodcalculates an image of the pattern (with, e.g., an aerial or resistmodel). Then, an attribute or metric of the imaged pattern (e.g., CDuniformity over an assumed error budget) is calculated, in a similarmanner as in FIG. 6. The assumed budget of errors may include mask error(size), dose and focus errors, and other important error contributors,depending on the nature of the problem. The value of the attributeinduced by the assumed budget may then be obtained for the first pitch.

Then, after obtaining the total variation of the attribute over anassumed budget of errors, the method proceeds to block 1040 where adetermination is made as to whether all the pitches have beenconsidered. If the result of the inquiry is negative, the methodproceeds back from block 1020, where a second pitch is selected, toblocks 1025 and 1030. If the result of the inquiry is positive, themethod proceeds to block 1040 where the metric is calculated for thefull pitch range. For example, if the metric is critical dimensionuniformity (CDU), the maximum and average variation of the criticaldimension uniformity (CDU) may be calculated.

Then, the method proceeds to block 1045 where a determination is made asto whether the flipping of the pixel group selected at block 1015improves or degrades the variation of the attribute. For example, if thefirst pixel group reduces the average and/or maximum of the metric(e.g., critical dimension uniformity (CDU) of the imaged pattern) overthe pitch range, the first pixel group is retained. By contrast, if thefirst pixel group increases the average and/or maximum of the metric(e.g., critical dimension uniformity of the imaged pattern) over thepitch range, the first pixel group is flipped back to its original state(i.e., if the first pixel group is “on”, it is turned “off”, and if thefirst pixel group is “off” it is turned “on”).

After determining the influence of the pixel group on the quality of theimaged pattern, the method proceeds to block 1050 where a determinationis made as to whether all the pixel groups have been considered. If theresult of the inquiry is negative, the method proceeds back to block1015 where a second pixel group is selected. If the result of theinquiry is positive, the method ends at block 1055. The user may chooseto loop through the pixel groups multiple times, or apply specificselection criteria, until a true convergence is obtained.

FIG. 11 a shows the variation of the critical dimension as a function ofpitch for a first illumination shape optimized in accordance with themethod of FIG. 10 (referred to as “pixel flip” in FIG. 11 a) and asecond illumination source optimized with a conventional optimizationmethod (referred to as “conventional” in FIG. 11 a) which used a similarmetric and the same calibrated model but a simple parametric sourceshape. FIGS. 11 b-c show, respectively, the first illumination shapeoptimized in accordance with the method of FIG. 10 and the secondillumination source optimized with a conventional optimization method.The pattern considered is a pattern of 65 nm holes arranged in 110, 130,150, 170, 190, 210 and 230 pitches. Calculations are made with a 20%attenuated phase shift mask, a 193 nm radiation wavelength and a 0.93numerical aperture. The metric or attribute considered to configure theillumination source is the critical dimension uniformity of the holes.The selected metric, response or attribute to gauge the quality of theillumination shape is shown in equation (4):

$\begin{matrix}{{CDU} = {\left( {\frac{\sum\limits_{i = 1}^{n}{CDU}_{i}}{n} + {CDU}_{worsthole}} \right)/2}} & (4)\end{matrix}$Where n represents the total number of holes.Calculations of the metric are made for an assumed budget of focus (0.12μm), dose (2%) and mask size (2 nm). The total CDU estimated for eachhole is obtained by adding in quadrature the influence of eachindividual budgeted error on that hole's CDU.

As can be seen in FIG. 11 a, the CD uniformity is significantly improvedthrough the entire pitch range when the illumination source is optimizedin accordance with the method of FIG. 10 (referred to as “pixel flip” inFIG. 11 a).

In an embodiment, a similar approach may be used to configure anillumination source for a plurality of different patterns. It will beappreciated that the multiple pitch problem is just an example of themore general case of multiple patterns. Thus, instead of performing thecalculations for different pitches of a same pattern, the calculationsmay be performed for different patterns in a similar manner as themethod of FIG. 10.

Optimization of the illumination conditions to print complex patternsmay also be determined in accordance with the method of FIG. 2. FIG. 12shows an example of a complex pattern 1200. This pattern includes aseries of holes 1201 arranged with different pitches. In oneimplementation, block 204 of FIG. 2 includes determining an initial doseto size for a selected hole at fixed bias, calculating the horizontaland vertical biases for a plurality of holes (e.g., holes 1-6 of FIG.12) to print all holes to target size, calculating the CD range inducedby an assumed budget of focus, dose and mask size errors and adding theCD errors in quadrature to obtain a total CD error. This process may beiterated, in a similar manner as in FIG. 6, over some or all the pixelgroups to refine the initial illumination shape.

In one implementation, the order of selection of the pixel groups may bechosen so as to provide a lithographic process that is substantiallyisofocal over a predetermined range of defocus. For some patterns,different parts of the illumination system bring different benefits. Forexample, for a pattern of small trenches, a small sigma illumination(i.e., a circular illumination located at the center of the illuminationsystem), provides good exposure latitude but poor depth of focus. Bycontrast, the outer portion of the illumination system generallyprovides a better depth of focus for this pattern. Therefore, it may bedesirable to consider both the influence of the outer and inner portionsof the illumination system when configuring or optimizing theillumination source. Additional information regarding the concept ofisofocal compensation may be gleaned, for example, in U.S. Pat. No.7,016,017, issued on Mar. 1, 2006, entitled “Lithographic Apparatus andMethod for Optimizing an Illumination Source Using IsofocalCompensation.” The contents of this application are incorporated hereinin their entirety by reference.

In order to configure the illumination source in accordance with oneembodiment of the invention, all the pixel groups are flipped “on”.Thus, the initial source corresponds to a full circular illuminationwith a normalized radius σ=1. Then, the pixel group positioned at themiddle of the illumination is first selected (i.e., it is turned “off”).For example, if the total number of pixel groups is 60 (4 symmetryplanes—Table 2), the first pixel group is pixel group no. 30. If thetotal number of pixel groups is 117 (2 symmetry planes—Table 1), thefirst pixel group is pixel group no. 58 or 59. Then, the influence ofthe first pixel group on the quality of the imaged pattern is determinedin a similar manner as in FIG. 6. That is, a lithographic metric (e.g.the variation of a critical dimension of the imaged pattern) iscalculated over an assumed budget of errors to determine whether thefirst pixel group should be retained (i.e., kept “off”) or flipped backto its original state (i.e., “on”). Then, the remaining pixel groups aresuccessively and alternatively interrogated/selected nearer the center(e.g. pixel group no. 29) then farther from the middle (e.g., pixelgroup no. 31). Tables 3 and 4 show examples of the order of selection ofthe pixel groups for a two symmetry plane configuration (table 3) and afour symmetry plane configuration (table 4), respectively.

TABLE 4 Pixel Selection Group 1 58 2 57 3 59 4 56 5 60 6 55 7 61 8 54 962 10 53 11 63 12 52 13 64 14 51 15 65 16 50 17 66 18 49 19 67 20 48 2168 22 47 23 69 24 46 25 70 26 45 27 71 28 44 29 72 30 43 31 73 32 42 3374 34 41 35 75 36 40 37 76 38 39 39 77 40 38 41 78 42 37 43 79 44 36 4580 46 35 47 81 48 34 49 82 50 33 51 83 52 32 53 84 54 31 55 85 56 30 5786 58 29 59 87 60 28 61 88 62 27 63 89 64 26 65 90 66 25 67 91 68 24 6992 70 23 71 93 72 22 73 94 74 21 75 95 76 20 77 96 78 19 79 97 80 18 8198 82 17 83 99 84 16 85 100 86 15 87 101 88 14 89 102 90 13 91 103 92 1293 104 94 11 95 105 96 10 97 106 98 9 99 107 100 8 101 108 102 7 103 109104 6 105 110 106 5 107 111 108 4 109 112 110 3 111 113 112 2 113 114114 1 115 115 116 116 117 117

TABLE 5 Pixel Selection Group 1 30 2 29 3 31 4 28 5 32 6 27 7 33 8 26 934 10 25 11 35 12 24 13 36 14 23 15 37 16 22 17 38 18 21 19 39 20 20 2140 22 19 23 41 24 18 25 42 26 17 27 43 28 16 29 44 30 15 31 45 32 14 3346 34 13 35 47 36 12 37 48 38 11 39 49 40 10 41 50 42 9 43 51 44 8 45 5246 7 47 53 48 6 49 54 50 5 51 55 52 4 53 56 54 3 55 57 56 2 57 58 58 159 59 60 60It will be appreciated that a different order of selection for the pixelgroups may be chosen in another embodiment of the invention.

Referring to FIG. 13 a, this figure shows the variation of exposurelatitude as a function of depth of focus for (a) a source configured inaccordance with the method of FIG. 2 using isofocal compensation, (b) acircular illumination source (σ_(r)=0.6) and (c) an annular illuminationsource (σ_(c)=0.6; σ_(r)=0.9). For reference, the source optimized inaccordance with the method of FIG. 2 using isofocal compensation isshown in FIG. 13 b. This source includes a small circular sigma pole andfour poles arranged along the X and Y pupil axes. Calculations are donefor a 75 nm trench, a 15 nm patterning device bias, a 6% attenuatedphase shift mask, a 193 nm radiation wavelength and a 0.93 numericalaperture. FIG. 13 c shows the pattern of 75 nm trenches 1300 used in thecalculation.

The circular illumination source (σ_(r)=0.6) and an annular illuminationsource are two potential illumination sources that would beconventionally selected to expose this trench. However, as can be seenin FIG. 13 a, the configuration or optimization of the illuminationsource in accordance with FIG. 2 using the principle of isofocalcompensation significantly improves the exposure latitude as compared tothe conventional sources.

It will be appreciated that pixel groups including illumination sourcepoints having a normalized radial position greater than 1, i.e.,σ_(r)>1, may also be considered to configure or optimize theillumination shape. For illumination source points having a normalizedradial position greater than 1 (σ_(r)>1), “normal” imaging cannot occurbecause the projection system PS does not transmit any zero^(th) orderdiffracted beam generated by the illumination beam B. However, imagingwith high diffraction orders may be possible, and the informationcontained in these high diffraction orders may be used beneficially toprint some features. This type of imaging may be referred to as imagingusing dark-field illumination, named analogously to dark fieldmicroscopy, in reference to the fact that the zero^(th) diffractionorder is not collected by the projection system. It will be appreciatedthat the concept “dark-field illumination” in the present application isdefined independently from the commonly used concepts of dark-field maskpatterns and bright-field mask patterns. It is proposed in oneembodiment of the present invention to configure or optimize theillumination shape with group pixels having a normalized radial positiongreater than 1, i.e., σ>1

It will also be appreciated that additional metrics, attributes,responses or parameters may be used in configuring or optimizing theillumination source using group pixel flipping. For example, in oneembodiment of the invention, consideration may be given to the influenceof the selected group pixel on the heating of an optical element (e.g.,a lens). Specifically, in one configuration, the incremental effect ofeach excluded pixel group on the heating of a selected optical elementmay be determined. Additional metrics, attributes, responses orparameters may also be used in other embodiments of the invention toconfigure the illumination shape.

It will be appreciated that the different acts involved in configuringthe illumination source may be executed according to machine executableinstructions or codes. These machine executable instructions may beembedded in a data storage medium, e.g., of a control unit of thelithographic apparatus. The control unit may include a processor that isconfigured to control the adjusting device AM and to modify thecross-sectional intensity distribution in the beam exiting theillumination system IL. The processor may be configured to execute theinstructions.

While much of the description has been in terms of optimization,optimization need not be performed all or part of the time or for allparts of the illumination and/or pattern/patterning device. For example,the source optimization may be performed completely or partially“sub-optimally” for expedience, due to imaging requirements, for partsof the patterning device/pattern, etc.

Software functionalities of a computer system involving programming,including executable codes, may be used to implement the above describedimaging model. The software code may be executable by a general-purposecomputer. In operation, the code and possibly the associated datarecords may be stored within a general-purpose computer platform. Atother times, however, the software may be stored at other locationsand/or transported for loading into an appropriate general-purposecomputer system. Hence, the embodiments discussed above involve one ormore software or computer products in the form of one or more modules ofcode carried by at least one machine-readable medium. Execution of suchcodes by a processor of the computer system enables the platform toimplement the functions in essentially the manner performed in theembodiments discussed and illustrated herein.

As used herein, terms such as computer or machine “readable medium”refer to any medium that participates in providing instructions to aprocessor for execution. Such a medium may take many forms, includingbut not limited to, non-volatile media, volatile media, and transmissionmedia. Non-volatile media include, for example, optical or magneticdisks, such as any of the storage devices in any computer(s) operatingas discussed above. Volatile media include dynamic memory, such as themain memory of a computer system. Physical transmission media includecoaxial cables, copper wire and fiber optics, including the wires thatcomprise a bus within a computer system. Carrier-wave transmission mediacan take the form of electric or electromagnetic signals, or acoustic orlight waves such as those generated during radio frequency (RF) andinfrared (IR) data communications. Common forms of computer-readablemedia therefore include, for example: a floppy disk, a flexible disk,hard disk, magnetic tape, any other magnetic medium, a CD-ROM, DVD, anyother optical medium, less commonly used media such as punch cards,paper tape, any other physical medium with patterns of holes, a RAM, aPROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, acarrier wave transporting data or instructions, cables or linkstransporting such a carrier wave, or any other medium from which acomputer can read or send programming codes and/or data. Many of theseforms of computer readable media may be involved in carrying one or moresequences of one or more instructions to a processor for execution.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced other than asdescribed. The description is not intended to limit the invention.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion,” respectively. The substrate referred toherein may be processed, before or after exposure, in, for example, atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The lithographic apparatus may also be of a type wherein a surface ofthe substrate is immersed in a liquid having a relatively highrefractive index, e.g., water, so as to fill a space between a finalelement of the projection system and the substrate. Immersion liquidsmay also be applied to other spaces in the lithographic apparatus, forexample between the patterning device and a first element of theprojection system. Immersion techniques are well known in the art forincreasing the numerical aperture of projection systems.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens,” where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g., semiconductor memory, magnetic or optical disk) havingsuch a computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to those skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

What is claimed is:
 1. A method for configuring an illumination sourceof a lithographic apparatus, the method comprising: dividing theillumination source into pixel groups, each pixel group including one ormore illumination source points; selecting an initial illumination shapeto expose a pattern, the initial illumination shape formed with at leastone pixel group; iteratively calculating a lithographic metric for aplurality of pixel groups, each iteration in said calculating performedfor a change of state of one pixel group of said plurality of pixelgroups, the change of the state of the one pixel group creating amodified illumination shape, the iteratively calculating comprisingadding said one pixel group to the initial illumination shape orremoving said one pixel group from the initial illumination shape tocreate the modified illumination shape, the modified illumination shapeincluding a) the at least one pixel group from the initial illuminationshape and said one pixel group in the illumination source or b) the atleast one pixel group from said initial illumination shape without saidone pixel group in the illumination source, subsequent to said adding orremoving, calculating the lithographic metric using the modifiedillumination shape to determine whether said one pixel group should beadded or removed; and adjusting the illumination shape based on theiterative results of calculations.
 2. The method of claim 1, whereinstates of the plurality of pixel groups are changed with a predeterminedorder or randomly.
 3. The method of claim 2, wherein the state of eachof the plurality of pixel groups is changed based on a radial or angularposition of each pixel group in the illumination source or both theradial and the angular position of each pixel group in the illuminationsource.
 4. The method of claim 2, wherein the order is based on anincrease or decrease of the radial position of the pixel groups.
 5. Themethod of claim 4, wherein the states of the plurality of pixel groupsare changed by alternatively changing (a) a state of a pixel grouphaving a radial position greater than that of a predetermined pixelgroup and (b) a state of a pixel group having a radial position smallerthan that of the predetermined pixel group.
 6. The method of claim 1,wherein the plurality of pixel groups includes all the pixel groups. 7.The method of claim 1, wherein the lithographic metric is a criticaldimension uniformity of the pattern, a process window, a dimension ofthe process window, MEEF, maximum NILS, or maximum NILS in defocus. 8.The method of claim 7, wherein a value of a critical dimensionuniformity as predicted by the lithographic metric is calibrated tomatch with a measured critical dimension uniformity.
 9. The method ofclaim 1, wherein the lithographic metric is calculated over an assumedbudget of errors.
 10. The method of claim 9, wherein the errors includea patterning device error, a dose error and a focus error.
 11. Themethod of claim 1, wherein the illumination source is divided into pixelgroups in a pupil plane of an illumination system.
 12. The method ofclaim 11, wherein the illumination source includes pixel groups having anormalized radial position σ with respect to a full aperture of theillumination system greater than
 1. 13. The method of claim 1, whereinthe illumination source is divided into pixel groups based on a symmetryof the pattern.
 14. The method of claim 1, wherein calculating thelithographic metric includes calculating an image of the patternilluminated by the modified illumination shape.
 15. The method of claim14, wherein the lithographic metric is calculated using aphotolithographic simulation.
 16. The method of claim 15, wherein thephotolithographic simulation is performed using a calibrated lumpparameter model or a full resist process.
 17. The method of claim 1,wherein calculating a lithographic metric includes calculating thelithographic metric for a plurality of pitches.
 18. The method of claim1, wherein the calculating includes determining whether the change ofthe state of the pixel group in the illumination source should beretained based on results of the metric.
 19. The method of claim 18,wherein the calculating includes repeating the changing, the calculatingof the lithographic metric and the determining for another pixel group.20. A method for configuring an illumination source of a lithographicapparatus, the method comprising: dividing the illumination source intopixel groups, each pixel group including one or more illumination sourcepoints; selecting an initial illumination shape to expose a pattern;calculating a plurality of responses for a plurality of changes in theillumination source, each of the plurality of changes effected by achange of state of a single pixel group, the calculating comprisingadding said single pixel group to the initial illumination shape orremoving said single pixel group from the initial illumination shape tocreate a modified illumination shape, the modified illumination shapeincluding a) the at least one pixel group from the initial illuminationshape and said single pixel group or b) the at least one pixel groupfrom said initial illumination shape without said single pixel group,subsequent to said changing, calculating a response using the modifiedillumination shape to determine whether said single pixel group shouldbe added or removed; and adjusting the illumination shape based on theplurality of responses.
 21. The method of claim 20, wherein calculatinga plurality of responses includes calculating a lithographic metric. 22.The method of claim 21, wherein calculating the lithographic metricincludes calculating an image of the pattern.
 23. The method of claim21, wherein the lithographic metric is a critical dimension uniformityof the pattern, a process window, a dimension of the process window,MEEF, maximum NILS, maximum NILS in defocus.
 24. The method of claim 21,wherein the lithographic metric is calculated over an assumed budget oferrors.
 25. The method of claim 24, wherein the errors include apatterning device error, a dose error and a focus error.
 26. The methodof claim 20, wherein the plurality of pixel groups are changed with apredetermined order or randomly.
 27. The method of claim 20, wherein thechange of state of the pixel group is effected based on its influence onheating of a projection lens, the projection lens adapted to project animage of the exposed pattern onto a substrate.
 28. A non-transitorycomputer product having machine executable instructions, theinstructions being executable by a machine to perform a method forconfiguring an illumination source of a lithographic apparatus, themethod comprising: dividing the illumination source into pixel groups,each pixel group including one or more illumination source points;selecting an initial illumination shape to expose a pattern, theillumination shape formed with at least one pixel group; iterativelycalculating a lithographic metric for a plurality of pixel groups, eachiteration in said calculating performed for a change of state of onepixel group of said plurality of pixel groups, the change of the stateof the one pixel group creating a modified illumination shape, theiteratively calculating comprising adding said one pixel group to theinitial illumination shape or removing said one pixel group from theinitial illumination shape to create the modified illumination shape,the modified illumination shape including a) the at least one pixelgroup from the initial illumination shape and said one pixel group inthe illumination source or b) the at least one pixel group from saidinitial illumination shape without said one pixel group in theillumination source, subsequent to said changing, calculating thelithographic metric using the modified illumination shape to determinewhether said one pixel group should be added or removed; and adjustingthe illumination shape based on the iterative results of calculations.29. A lithographic apparatus comprising: an illumination systemconfigured to condition a beam of radiation; a substrate tableconfigured to hold a substrate; a projection system configured toproject a beam of radiation patterned by a patterning device onto asurface of the substrate; and a processor configured to perform a methodfor configuring an illumination source of a lithographic apparatus, themethod comprising dividing the illumination source into pixel groups,each pixel group including one or more illumination source points;selecting an initial illumination shape to expose a pattern, theillumination shape formed with at least one pixel group; iterativelycalculating a lithographic metric for a plurality of pixel groups, eachiteration in said calculating performed for a change of state of onepixel group of said plurality of pixel groups, the change of the stateof the one pixel group creating a modified illumination shape, theiteratively calculating comprising adding said one pixel group to theinitial illumination shape or removing said one pixel group from theinitial illumination shape to create the modified illumination shape,the modified illumination shape including a) the at least one pixelgroup from the initial illumination shape and said one pixel group or b)the at least one pixel group from said initial illumination shapewithout said one pixel group, subsequent to said changing, calculatingthe lithographic metric using the modified illumination shape todetermine whether said one pixel group should be added or removed; andadjusting the illumination shape based on the iterative results ofcalculations.